Thin film optical measurement system and method with calibrating ellipsometer

ABSTRACT

An optical measurement system for evaluating a reference sample that has at least a partially known composition. The optical measurement system includes a reference ellipsometer and at least one non-contact optical measurement device. The reference ellipsometer includes a light generator, an analyzer and a detector. The light generator generates a beam of quasi-monochromatic light having a known wavelength and a known polarization for interacting with the reference sample. The beam is directed at a non-normal angle of incidence relative to the reference sample to interact with the reference sample. The analyzer creates interference between the S and P polarized components in the light beam after the light beam has interacted with reference sample. The detector measures the intensity of the light beam after it has passed through the analyzer. A processor determines the polarization state of the light beam entering the analyzer from the intensity measured by the detector, and determines an optical property of the reference sample based upon the determined polarization state, the known wavelength of light from the light generator and the composition of the reference sample. The processor also operates the optical measurement device to measure an optical parameter of the reference sample. The processor calibrates the optical measurement device by comparing the measured optical parameter from the optical measurement device to the determined optical property from the reference ellipsometer.

RELATED APPLICATION

[0001] The present application is a divisional of U.S. application Ser.No. 09/886,514, filed Jun. 21, 2001, which is in turn a continuation ofU.S. application Ser. No. 09/247,121, filed Feb. 8, 1999, now U.S. Pat.No. 6,304,326, which is in turn a continuation of U.S. application Ser.No. 09/098,880, filed Jun. 17, 1998, now U.S. Pat. No. 5,900,939, whichis in turn a continuation of U.S. application Ser. No. 08/890,697, filedJul. 11, 1997, now U.S. Pat. No. 5,798,837.

FIELD OF THE INVENTION

[0002] The present invention relates to optical analyzers, and moreparticularly to a thin film optical measurement system having acalibrating ellipsometer.

BACKGROUND OF THE INVENTION

[0003] There is considerable interest in developing systems foraccurately measuring the thickness and/or composition of thin films. Theneed is particularly acute in the semiconductor manufacturing industrywhere the thickness of these thin film oxide layers on semiconductorsubstrates is measured. To be useful, the measurement system must beable to determine the thickness and/or composition of films with a highdegree of accuracy. The preferred measurement systems rely onnon-contact, optical measurement techniques, which can be performedduring the semiconductor manufacturing process without damaging thewafer sample. Such optical measurement techniques include directing aprobe beam to the sample, and measuring one or more optical parametersof the reflected probe beam.

[0004] In order to increase measurement accuracy and to gain additionalinformation about the target sample, multiple optical measuring devicesare incorporated into a single composite optical measurement system. Forexample, the present assignee has marketed a product called OPTI-PROBE,which incorporates several optical measurement devices, including a BeamProfile Reflectometer (BPR), a Beam Profile Ellipsometer (BPE), and aBroadband Reflective Spectrometer (BRS). Each of these devices measuresparameters of optical beams reflected by, or transmitted through, thetarget sample. The BPR and BPE devices utilize technology described inU.S. Pat. Nos. 4,999,014 and 5,181,080 respectively, which areincorporated herein by reference.

[0005] The composite measurement system mentioned above combines themeasured results of each of the measurement devices to precisely derivethe thickness and composition of the thin film and substrate of thetarget sample. However, the accuracy of the measured results dependsupon precise initial and periodic calibration of the measurement devicesin the optical measurement system. Further, recently developedmeasurement devices have increased sensitivity to more accuratelymeasure thinner films and provide additional information about film andsubstrate composition. These newer systems require very accurate initialcalibration. Further, heat, contamination, optical damage, alignment,etc., that can occur over time in optical measurement devices, affectthe accuracy of the measured results. Therefore, periodic calibration isnecessary to maintain the accuracy of the composite optical measurementsystem.

[0006] It is known to calibrate optical measurement devices by providinga reference sample having a known substrate, with a thin film thereonhaving a known composition and thickness. The reference sample is placedin the measurement system, and each optical measurement device measuresthe optical parameters of the reference sample, and is calibrated usingthe results from the reference sample and comparing them to the knownfilm thickness and composition. A common reference sample is a “nativeoxide” reference sample, which is a silicon substrate with an oxidelayer formed thereon having a known thickness (about 20 angstroms).After fabrication, the reference sample is kept in a non-oxygenenvironment to minimize any further oxidation and contamination thatchanges the thickness of the reference sample film away from the knownthickness, and thus reduces the effectiveness of the reference samplefor accurate calibration. The same reference sample can be reused toperiodically calibrate the measurement system. However, if and when theamount of oxidation or contamination of the reference sample changes thefilm thickness significantly from the known thickness, the referencesample must be discarded.

[0007] For many optical measurement devices, reference samples withknown thicknesses have been effective for system calibration. Oxidationand contamination that routinely occurs over time with reference samplesis tolerable because the film thickness change resulting from theoxidation/contamination is relatively insignificant compared -to theoverall thickness of the film (around 100 angstroms). However, newultra-sensitive optical measurement systems have been recently developedthat can measure film layers with thicknesses less than 10 angstroms.These systems require reference samples having film thicknesses on theorder of 20 angstroms for accurate calibration. For such thin filmreference samples, however, the changes in film layer thicknessresulting from even minimal oxidation or contamination are significantcompared to the overall “known” film layer thickness, and result insignificant calibration error. Therefore, it is extremely difficult, ifnot impossible, to provide a native oxide reference sample with a knownthickness that is stable enough over time to be used for periodiccalibration of ultra-sensitive optical measurement systems.

[0008] There is a need for a calibration method for ultra-sensitiveoptical measurement devices that can utilize a reference sample thatdoes not have a stable or known film thickness.

SUMMARY OF THE INVENTION

[0009] The present invention is a thin film optical measurement systemwith a wavelength stable calibration ellipsometer that preciselydetermines the thickness of a film on a reference sample. The measuredresults from the calibration ellipsometer are used to calibrate otheroptical measurement devices in the thin film optical measurement system.By not having to supply a reference sample with a predetermined knownfilm thickness, a reference sample having a film with a knowncomposition can be repeatedly used to calibrate ultra-sensitive opticalmeasurement devices, even if oxidation or contamination of the referencesample changes the thickness of the film over time.

[0010] The calibration reference ellipsometer uses a reference samplethat has at least a partially known composition to calibrate at leastone other non-contact optical measurement device. The referenceellipsometer includes a light generator that generates aquasi-monochromatic beam of light having a known wavelength and a knownpolarization for interacting with the reference sample. The beam isdirected at a non-normal angle of incidence relative to the referencesample to interact with the reference sample. An analyzer createsinterference between S and P polarized components in the light beamafter the light beam has interacted with reference sample. A detectormeasures the intensity of the light after the beam has passed throughthe analyzer. A processor determines the polarization state of the lightbeam entering the analyzer from the intensity measured by the detector.The processor then determines optical properties of the reference samplebased upon the determined polarization state, the known wavelength oflight from the light generator and the at least partially knowncomposition of the reference sample. Wherein the processor operates atleast one other non-contact optical measurement device that measures anoptical parameter of the reference sample. The processor calibrates theother optical measurement device by comparing the measured opticalparameter from the other optical measurement device to the determinedoptical property from the reference ellipsometer.

[0011] Other aspects and features of the present invention will becomeapparent by a review of the specification, claims and appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a plan view of a composite optical measurement systemwith the calibration ellipsometer of the present invention.

[0013]FIG. 2 is a side cross-sectional view of the reflective lens usedwith the present invention.

[0014]FIG. 3 is a plan view of an alternate embodiment of the lightsource for the calibration ellipsometer of the present invention.

[0015]FIG. 4 is a plan view of the composite optical measurement systemwith multiple compensators in the calibration ellipsometer of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The present invention is a composite thin film opticalmeasurement system 1 having a wavelength stable reference ellipsometer 2that is used, in conjunction with a reference sample 4 having asubstrate 6 and thin film 8 with known compositions, to calibratenon-contact optical measurement devices contained in the composite thinfilm optical measurement system 1.

[0017]FIG. 1 illustrates the composite optical measurement system 1 thathas been developed by the present assignees, which includes fivedifferent non-contact optical measurement devices and the referenceellipsometer 2 of the present invention.

[0018] Composite optical measurement system, 1 includes a Beam ProfileEllipsometer (BPE) 10, a Beam Profile Reflectometer (BPR) 12, aBroadband Reflective Spectrometer (BRS) 14, a Deep Ultra VioletReflective Spectrometer (DUV) 16, and a Broadband SpectroscopicEllipsometer (BSE) 18. These five optical measurement devices utilize asfew as two optical sources: laser 20 and white light source 22. Laser 20generates a probe beam 24, and white light source 22 generates probebeam 26 (which is collimated by lens 28 and directed along the same pathas probe beam 24 by mirror 29). Laser 20 ideally is a solid state laserdiode from Toshiba Corp. which emits a linearly polarized 3 mW beam at673 nm. White light source 22 is ideally a deuterium-tungsten lamp thatproduces a 200 mW polychromatic beam that covers a spectrum of 200 nm to800 nm. The probe beams 24/26 are reflected by mirror 30, and passthrough mirror 42 to sample 4.

[0019] The probe beams 24/26 are focused onto the surface of the samplewith a lens 32 or lens 33. In the preferred embodiment, two lenses 32/33are mounted in a turret (not shown) and are alternatively movable intothe path of probe beams 24/26. Lens 32 is a spherical, microscopeobjective lens with a high numerical aperture (on the order of 0.90 NA)to create a large spread of angles of incidence with respect to thesample surface, and to create a spot size of about one micron indiameter. Lens 33 is illustrated in FIG. 2, and is a reflective lenshaving a lower numerical aperture (on the order of 0.4 NA) and capableof focusing deep UV light to a spot size of about 10-15 microns.

[0020] Beam profile ellipsometry (BPE) is discussed in U.S. Pat. No.5,181,080, issued Jan. 19, 1993, which is commonly owned by the presentassignee and is incorporated herein by reference. BPE 10 includes aquarter wave plate 34, polarizer 36, lens 38 and a quad detector 40. Inoperation, linearly polarized probe beam 24 is focused onto sample 4 bylens 32. Light reflected from the sample surface passes up through lens32, through mirrors 42, 30 and 44, and directed into BPE 10 by mirror46. The position of the rays within the reflected probe beam correspondto specific angles of incidence with respect to the sample's surface.Quarter-wave plate 34 retards the phase of one of the polarizationstates of the beam by 90 degrees. Linear polarizer 36 causes the twopolarization states of the beam to interfere with each other. Formaximum signal, the axis of the polarizer 36 should be oriented at anangle of 45 degrees with respect to the fast and slow axis of thequarter-wave plate 34. Detector 40 is a quad-cell detector with fourradially disposed quadrants that each intercept one quarter of the probebeam and generate a separate output signal proportional to the power ofthe portion of the probe beam striking that quadrant. The output signalsfrom each quadrant are sent to a processor 48. As discussed in the U.S.Pat. No. 5,181,080 patent , by monitoring the change in the polarizationstate of the beam, ellipsometric information, such as ψ and Δ, can bedetermined. To determine this information, the processor 48 takes thedifference between the sums of the output signals of diametricallyopposed quadrants, a value which varies linearly with film thickness forvery thin films.

[0021] Beam profile reflectometry (BPR) is discussed in U.S. Pat. No.4,999,014, issued on Mar. 12, 1991, which is commonly owned by thepresent assignee and is incorporated herein by reference. BPR 12includes a lens 50, beam splitter 52 and two linear detector arrays 54and 56 to measure the reflectance of the sample. In operation, linearlypolarized probe beam 24 is focused onto sample 4 by lens 32, withvarious rays within the beam striking the sample surface at a range ofangles of incidence. Light reflected from the sample surface passes upthrough lens 32, through mirrors 42 and 30, and directed into BPR 12 bymirror 44. The position of the rays within the reflected probe beamcorrespond to specific angles of incidence with respect to the sample'ssurface. Lens 50 spatially spreads the beam two-dimensionally. Beamsplitter 52 separates the S and P components of the beam, and detectorarrays 54 and 56 are oriented orthogonal to each other to isolateinformation about S and P polarized light. The higher angles ofincidence rays will fall closer to the opposed ends of the arrays. Theoutput from each element in the diode arrays will correspond todifferent angles of incidence. Detector arrays 54/56 measure theintensity across the reflected probe beam as a function of the angle ofincidence with respect to the sample surface. The processor 48 receivesthe output of the detector arrays 54/56, and derives the thickness andrefractive index of the thin film layer 8 based on these angulardependent intensity measurements by utilizing various types of modelingalgorithms. Optimization routines which use iterative processes such asleast square fitting routines are typically employed. One example ofthis type of optimization routine is described in “MultiparameterMeasurements of Thin Films Using Beam-Profile Reflectivity,” Fanton, et.al., Journal of Applied Physics, Vol. 73, No. 11, p.7035, 1993.

[0022] Broadband reflective spectrometer (BRS) 14 simultaneously probesthe sample 4 with multiple wavelengths of light. BRS 14 uses lens 32 andincludes a broadband spectrometer 58 which can be of any type commonlyknown and used in the prior art. The spectrometer 58 shown in FIG. 1includes a lens 60, aperture 62, dispersive element 64 and detectorarray 66. During operation, probe beam 26 from white light source 22 isfocused onto sample 4 by lens 32. Light reflected from the surface ofthe sample passes up through lens 32, and is directed by mirror 42(through mirror 84) to spectrometer 58. The lens 60 focuses the probebeam through aperture 62, which defines a spot in the field of view onthe sample surface to analyze. Dispersive element 64, such as adiffraction grating, prism or holographic plate, angularly disperses thebeam as a function of wavelength to individual detector elementscontained in the detector array 66. The different detector elementsmeasure the optical intensities of the different wavelengths of lightcontained in the probe beam, preferably simultaneously.. Alternately,detector 66 can be a CCD camera, or a photomultiplier with suitablydispersive or otherwise wavelength selective optics. It should be notedthat a monochrometer could be used to measure the different wavelengthsserially (one wavelength at a time) using a single detector element.Further, dispersive element 64 can also be configured to disperse thelight as a function of wavelength in one direction, and as a function ofthe angle of incidence with respect to the sample surface in anorthogonal direction, so that simultaneous measurements as a function ofboth wavelength and angle of incidence are possible. Processor 48processes the intensity information measured by the detector array 66.

[0023] Deep ultra violet reflective spectrometry (DUV) simultaneouslyprobes the sample with multiple wavelengths of ultra-violet light. DUV16 uses the same spectrometer 58 to analyze probe beam 26 as BRS 14,except that DUV 16 uses the reflective lens 33 instead of focusing lens32. To operate DUV 16, the turret containing lenses 32/33 is rotated sothat reflective lens 33 is aligned in probe beam 26. The reflective lens33 is necessary because solid objective lenses cannot sufficiently focusthe UV light onto the sample.

[0024] Broadband spectroscopic ellipsometry (BSE) is discussed in U.S.Pat. No. 5,877,859, which is commonly owned by the present assignee andis incorporated herein by reference. BSE (18) includes a polarizer 70,focusing mirror 72, collimating mirror 74, rotating compensator 76, andanalyzer 80. In operation, mirror 82 directs at least part of probe beam26 to polarizer 70, which creates a known polarization state for theprobe beam, preferably a linear polarization. Mirror 72 focuses the beamonto the sample surface at an oblique angle, ideally on the order of 70degrees to the normal of the sample surface. Based upon well knownellipsometric principles, the reflected beam will generally have a mixedlinear and circular polarization state after interacting with thesample, based upon the composition and thickness of the sample's film 8and substrate 6. The reflected beam is collimated by mirror 74, whichdirects the beam to the rotating compensator 76. Compensator 76introduces a relative phase delay δ (phase retardation) between a pairof mutually orthogonal polarized optical beam components. Compensator 8is rotated at an angular velocity ω about an axis substantially parallelto the propagation direction of the beam, preferably by an electricmotor 78. Analyzer 80, preferably another linear polarizer, mixes thepolarization states incident on it. By measuring the light transmittedby analyzer 80, the polarization state of the reflected probe beam canbe determined. Mirror 84 directs the beam to spectrometer 58, whichsimultaneously measures the intensities of the different wavelengths oflight in the reflected probe beam that pass through thecompensator/analyzer combination. Processor 48 receives the output ofthe detector 66, and processes the intensity information measured by thedetector 66 as a function of wavelength and as a function of the azimuth(rotational) angle of the compensator 76 about its axis of rotation, tosolve the ellipsometric values ψ and Δ as described in U.S. Pat. No.5,877,859.

[0025] Detector/camera 86 is positioned above mirror 46, and can be usedto view reflected beams off of the sample 4 for alignment and focuspurposes.

[0026] In order to calibrate BPE 10, BPR 12, BRS 14, DUV 16, and BSE 18,the composite optical measurement system 1 includes the wavelengthstable calibration reference ellipsometer 2 used in conjunction with areference sample 4. Ellipsometer 2 includes a light source 90, polarizer92, lenses 94 and 96, rotating compensator 98, analyzer 102 and detector104.

[0027] Light source 90 produces a quasi-monochromatic probe beam 106having a known stable wavelength and stable intensity. This can be donepassively, where light source 90 generates a very stable outputwavelength which does not vary over time (i.e. varies less than 1%).Examples of passively stable light sources are a helium-neon laser, orother gas discharge laser systems. Alternately, a non-passive system canbe used as illustrated in FIG. 3 where the light source 90 includes alight generator 91 that produces light having a wavelength that is notprecisely known or stable over time, and a monochrometer 93 thatprecisely measures the wavelength of light produced by light generator91. Examples of such light generators include laser diodes, orpolychromatic light sources used in conjunction with a color filter suchas a grating. In either case, the wavelength of beam 106, which is aknown constant or measured by monochrometer 93, is provided to processor48 so that ellipsometer 2 can accurately calibrate the opticalmeasurement devices in system 1.

[0028] The beam 106 interacts with polarizer 92 to create a knownpolarization state. In the preferred embodiment, polarizer 92 is alinear polarizer made from a quartz Rochon prism, but in general thepolarization does not necessarily have to be linear, nor even complete.Polarizer 92 can also be made from calcite. The azimuth angle ofpolarizer 92 is oriented so that the plane of the electric vectorassociated with the linearly polarized beam exiting from the polarizer92 is at a known angle with respect to the plane of incidence (definedby the propagation direction of the beam 106 and the normal to thesurface of sample 4). The azimuth angle is preferably selected to be onthe order of 30 degrees because the sensitivity is optimized when thereflected intensities of the P and S polarized components areapproximately balanced. It should be noted that polarizer 92 can beomitted if the light source 90 emits light with the desired knownpolarization state.

[0029] The beam 106 is focused onto the sample 4 by lens 94 at anoblique angle. For calibration purposes, reference sample 4 ideallyconsists of a thin oxide layer 8 having a thickness d, formed on asilicon substrate 6. However, in general, the sample 4 can be anyappropriate substrate of known composition, including a bare siliconwafer, and silicon wafer substrates having one or more thin filmsthereon. The thickness d of the layer 8 need not be known, or beconsistent between periodic calibrations. The useful light from probebeam 106 is the light reflected by the sample 4 symmetrically to theincident beam about the normal to the sample surface. It is notedhowever that the polarization state of nonspecularly scattered radiationcan be determined by the method of the present invention as well. Thebeam 106 is ideally incident on sample 4 at an angle on the order of 70degrees to the normal of the sample surface because sensitivity tosample properties is maximized in the vicinity of the Brewster orpseudo-Brewster angle of a material. Based upon well known ellipsometricprinciples, the reflected beam will generally have a mixed linear andcircular polarization state after interacting with the sample, ascompared to the linear polarization state of the incoming beam. Lens 96collimates beam 106 after its reflection off of the sample 4.

[0030] The beam 106 then passes through the rotating compensator(retarder) 98, which introduces a relative phase delay δ (phaseretardation) between a pair of mutually orthogonal polarized opticalbeam components. The amount of phase retardation is a function of thewavelength, the dispersion characteristics of the material used to formthe compensator, and the thickness of the compensator. Compensator 98 isrotated at an angular velocity co about an axis substantially parallelto the propagation direction of beam 106, preferably by an electricmotor 100. Compensator 98 can be any conventional wave-platecompensator, for example those made of crystal quartz. The thickness andmaterial of the compensator 98 are selected such that a desired phaseretardation of the beam is induced. In the preferred embodiment,compensator 98 is a bi-plate compensator constructed of two parallelplates of anisotropic (usually birefringent) material, such as quartzcrystals of opposite handedness, where the fast axes of the two platesare perpendicular to each other and the thicknesses are nearly equal,differing only by enough to realize a net first-order retardation forthe wavelength produced by the light source 90.

[0031] Beam 106 then interacts with analyzer 102, which serves to mixthe polarization states incident on it. In this embodiment, analyzer 102is another linear polarizer, preferably oriented at an azimuth angle of45 degrees relative to the plane of incidence. However, any opticaldevice that serves to appropriately mix the incoming polarization statescan be used as an analyzer. The analyzer 102 is preferably a quartzRochon or Wollaston prism. The rotating compensator 98 changes thepolarization state of the beam as it rotates such that the lighttransmitted by analyzer 102 is characterized by: $\begin{matrix}\begin{matrix}{{I(t)} = \quad {\left( {1/2} \right)\left\lbrack {{{E_{x}}^{2}\left( {1 + {\cos^{2}\left( {\delta/2} \right)} + {{E_{y}}^{2}{\sin^{2}\left( {\delta/2} \right)}}} \right\rbrack} -} \right.}} \\{\quad {{{{Im}\left( {E_{x}E_{y}^{*}} \right)}\sin \quad \delta \quad \sin \quad \left( {2\quad \omega \quad t} \right)} +}} \\{\quad {{{{Re}\left( {E_{x}E_{y}^{*}} \right)}{\sin^{2}\left( {\delta/2} \right)}\sin \quad \left( {4\quad \omega \quad t} \right)} +}} \\{\quad {\left( {1/2} \right)\left( {{E_{x}}^{2} - {E_{y}}^{2}} \right){\sin^{2}\left( {\delta/2} \right)}{\cos \left( {4\quad \omega \quad t} \right)}}}\end{matrix} & (1) \\{\quad {{= \quad {a_{0} + {b_{2}\sin \quad \left( {2\quad \omega \quad t} \right)} + {a_{4}\cos \quad \left( {4\quad \omega \quad t} \right)} + {b_{4}\sin \quad \left( {4\quad \omega \quad t} \right)}}},}\quad} & (2)\end{matrix}$

[0032] where E_(x) and E_(y) are the projections of the incidentelectric field vector parallel and perpendicular, respectively, to thetransmission axis of the analyzer, δ is the phase retardation of thecompensator, and ω is the angular rotational frequency of thecompensator.

[0033] For linearly polarized light reflected at non-normal incidencefrom the specular sample, we have

E_(x)=r_(p) cos P   (3a)

E_(y)=r_(s) sin P   (3b)

[0034] where P is the azimuth angle of the incident light with respectto the plane of incidence. The coefficients a_(o), b₂, a₄, and b₄ can becombined in various ways to determine the complex reflectance ratio:

r _(p) /r _(s)=tan ψe ^(iΔ).   (4)

[0035] It should be noted that the compensator 98 can be located eitherbetween the sample 4 and the analyzer 102 (as shown in FIG. 1), orbetween the sample 4 and the polarizer 92, with appropriate and wellknown minor changes to the equations. It should also be noted thatpolarizer 70, lenses 94/96, compensator 98 and polarizer 102 are alloptimized in their construction for the specific wavelength of lightproduced by light source 90, which maximizes the accuracy ofellipsometer 2.

[0036] Beam 106 then enters detector 104, which measures the intensityof the beam passing through the compensator/analyzer combination. Theprocessor 48 processes the intensity information measured by thedetector 104 to determine the polarization state of the light afterinteracting with the analyzer, and therefore the ellipsometricparameters of the sample. This information processing includes measuringbeam intensity as a function of the azimuth (rotational) angle of thecompensator about its axis of rotation. This measurement of intensity asa function of compensator rotational angle is effectively a measurementof the intensity of beam 106 as a function of time, since thecompensator angular velocity is usually known and a constant.

[0037] By knowing the composition of reference sample 4, and by knowingthe exact wavelength of light generated by light source 90, the opticalproperties of reference sample 4, such as film thickness d, refractiveindex and extinction coefficients, etc., can be determined byellipsometer 2. If the film is very thin, such as less than or equal toabout 20 angstroms, the thickness d can be found to first order in d/λby solving $\begin{matrix}{{\frac{\rho - \rho_{o}}{\rho_{o}} = {\frac{4\quad \pi \quad {id}\quad \cos \quad \theta}{\lambda}\frac{ɛ_{s}\left( {ɛ_{s} - ɛ_{o}} \right)\left( {ɛ_{o} - ɛ_{a}} \right)}{{ɛ_{o}\left( {ɛ_{s} - ɛ_{a}} \right)}\left( {{ɛ_{s}\cot^{2}\theta} - ɛ_{a}} \right)}}},} & (5) \\{where} & \quad \\{\rho_{o} = {\tan \quad \Psi_{o}^{\quad \Delta_{o}}}} & (6) \\{\quad {= \quad \frac{{\sin^{2}\theta} - {\cos \quad \theta \quad \left( {{ɛ_{s}/ɛ_{a}} - {\sin^{2}\theta}} \right)^{1/2}}}{{\sin^{2}\theta} + {\cos \quad \theta \quad \left( {{ɛ_{s}/ɛ_{a}} - {\sin^{2}\theta}} \right)^{1/2}}}}} & (7)\end{matrix}$

[0038] which is the value of ρ=tan ψe^(iΔ) for d=0. Here, ρ=wavelengthof light; and ε_(s), ε_(o) and ε_(a) are the dielectric functions of thesubstrate, thin oxide film, and ambient, respectively, and θ is theangle of incidence.

[0039] If the film thickness d is not small, then it can be obtained bysolving the equations

ρ=r _(p) /r _(s), where   (8)

[0040] $\begin{matrix}{r_{p} = \frac{r_{p,{oa}} + {Zr}_{p,{so}}}{1 + {{Zr}_{p,{oa}}r_{p,{so}}}}} & (9) \\{r_{s} = \frac{r_{s,{oa}} + {Zr}_{s,{so}}}{1 + {{Zr}_{s,{oa}}r_{s,{so}}}}} & (10)\end{matrix}$

[0041] and where

Z=e^(2ik d),   (11)

ck _(o) _(^(⊥)) /ω=n _(o) _(^(⊥)) =(ε_(o)/ε_(a)−sin²θ)^(1/2)   (12)

[0042] $\begin{matrix}{r_{s,{so}} = \frac{n_{o\bot} - n_{s\bot}}{n_{o\bot} + n_{s\bot}}} & (13) \\{r_{s,{oa}} = \frac{n_{a\bot} - n_{o\bot}}{n_{a\bot} + n_{o\bot}}} & (14) \\{r_{p,{so}} = \frac{{ɛ_{s}n_{o\bot}} - {ɛ_{o}n_{s\bot}}}{{ɛ_{s}n_{o\bot}} + {ɛ_{o}n_{s\bot}}}} & (15) \\{r_{p,{oa}} = \frac{{ɛ_{o}n_{a\bot}} - {ɛ_{a}n_{o\bot}}}{{ɛ_{o}n_{a\bot}} + {ɛ_{a}n_{o\bot}}}} & (16)\end{matrix}$

[0043] and in general

n _(j) _(^(⊥)) =(ε_(j)−ε_(a)sin²θ)^(1/2),   (17)

[0044] where j is s or a. These equations generally have to be solvednumerically for d and n_(o) simultaneously, using ε_(s), ε_(a), λ, andθ, which are known.

[0045] Once the thickness d of film 8 has been determined byellipsometer 2, then the same sample 4 is probed by the other opticalmeasurement devices BPE 10, BPR 12, BRS 14, DUV 16, and BSE 18 whichmeasure various optical parameters of the sample 4. Processor 48 thencalibrates the processing variables used to analyze the results fromthese optical measurement devices so that they produce accurate results.For each of these measurement devices, there are system variables thataffect the measured data and need to be accounted for before an accuratemeasurement of other samples can be made. In the case of BPE 10, themost significant variable system parameter is the phase shift thatoccurs due to the optical elements along the BPE optical path.Environmental changes to these optical elements result in an overalldrift in the ellipsometric parameter Δ, which then translates into asample thickness drift calculated by the processor 48 from BPE 10. Usingthe measured optical parameters of BPE 10 on reference sample 4, andusing Equation 5 and the thickness of film 8 as determined fromcalibration ellipsometer 2, the processor 48 calibrates BPE 10 byderiving a phase offset which is applied to measured results from BPE 10for other samples, thereby establishing an accurate BPE measurement. ForBSE 18, multiple phase offsets are derived for multiple wavelengths inthe measured spectrum.

[0046] For the remaining measurement devices, BPR 12, BRS 14 and DUV 16,the measured reflectances can also be, affected by environmental changesto the optical elements in the beam paths. Therefore, the reflectancesR_(ref) measured by BPR 12, BRS 14 and DUV 16 for the reference sample 4are used, in combination with the measurements by ellipsometer 2, tocalibrate these systems. Equations 9-17 are used to calculate theabsolute reflectances R^(c) _(ref) of reference sample 4 from themeasured results of ellipsometer 2. All measurements by the BPR/BRS/DUVdevices of reflectance (R_(s)) for any other sample are then scaled byprocessor 48 using the normalizing factor in equation 18 below to resultin accurate reflectances R derived from the BPR, BRS and DUV devices:

R=R _(s)(R ^(c) _(ref) /R _(ref))   (18)

[0047] In the above described calibration techniques, all systemvariables affecting phase and intensity are determined and compensatedfor using the phase offset and reflectance normalizing factor discussedabove, thus rendering the optical measurements made by these calibratedoptical measurement devices absolute.

[0048] The above described calibration techniques are based largely uponcalibration using the derived thickness d of the thin film. However,calibration using ellipsometer 2 can be based upon any of the opticalproperties of the reference sample that are measurable or determinableby ellipsometer 2 and/or are otherwise known, whether the sample has asingle film thereon, has multiple films thereon, or even has no filmthereon (bare sample).

[0049] The advantage of the present invention is that a reference samplehaving no thin film thereon, or having thin film thereon with an unknownthickness which may even vary slowly over time, can be repeatedly usedto accurately calibrate ultra-sensitive optical measurement devices.

[0050] The output of light source 90 can also be used to calibrate thewavelength measurements made by spectrometer 58. The sample 4 can betipped, or replaced by a tipped mirror, to direct beam 106 up to mirror42 and to dispersion element 64. By knowing the exact wavelength oflight produced by light source 90, processor 48 can calibrate the outputof detector 66 by determining which pixel(s) corresponds to thatwavelength of light.

[0051] It should be noted that the calibrating ellipsometer 2 of thepresent invention is not limited to the specific rotating compensatorellipsometer configuration discussed above. The scope of the presentinvention includes any ellipsometer configuration in conjunction withthe light source 90 (having a known wavelength) that measures thepolarization state of the beam after interaction with the sample andprovides the necessary information about sample 4 for calibratingnon-contact optical measurement devices.

[0052] For example, another ellipsometric configuration is to rotatepolarizer 92 or analyzer 100 with motor 100, instead of rotating thecompensator 98. The above calculations for solving for thickness d stillapply.

[0053] In addition, null ellipsometry, which uses the same elements asellipsometer 2 of FIG. 1, can be used to determine the film thickness dfor calibration purposes. The ellipsometric information is derived byaligning the azimuthal angles of these elements until a null or minimumlevel intensity is measured by the detector 104. In the preferred nullellipsometry embodiment, polarizers 92 and 102 are linear polarizers,and compensator 98 is a quarter-wave plate. Compensator 98 is aligned sothat its fast axis is at an azimuthal angle of 45 degrees relative tothe plane of incidence of the sample 4. Polarizer 92 has a transmissionaxis that forms an azimuthal angle P relative to the plane of incidence,and polarizer 102 has a transmission axis that forms an azimuthal angleA relative to the plane of incidence. Polarizers 92 and 102 are rotatedabout beam 106 such that the light is completely extinguished(minimized) by the analyzer 102. In general, there are two polarizer92/102 orientations (P₁, A₁) and (P₂, A₂) that satisfy this conditionand extinguish the light. With the compensator inducing a 90 degreephase shift and oriented with an azimuthal angle at 45 degree relativeto the plane of incidence, we have:

P ₂ =P ₁±π/2   (19)

A ₂=−A₁   (20)

ψ=A ₁≧0   (21)

[0054] (where A₁ is the condition for which A is positive).

Δ=2P ₁+π/2   (22)

[0055] which, when combined with equations 5-10, allows the processor tosolve for thickness d.

[0056] Null ellipsometry is very accurate because the results dependentirely on the measurement of mechanical angles, and are independent ofintensity. Null ellipsometry is further discussed by R. M. A. Azzam andN. M. Bashara, in Ellipsometry and Polarized Light (North-Holland,Amsterdam, 1977); and by D. E. Aspnes, in Optical Properties of Solids:New Developments, ed. B. O. Seraphin (North-Holland, Amsterdam, 1976),p. 799.

[0057] It is also conceivable to omit compensator 98 from ellipsometer2, and use motor 100 to rotate polarizer 92 or analyzer 102. Either thepolarizer 92 or the analyzer 102 is rotated so that the detector signalcan be used to accurately measure the linear polarization component ofthe reflected beam. Then, the circularly polarized component is inferredby assuming that the beam is totally polarized, and what is not linearlypolarized must be circularly polarized. Such an ellipsometer, commonlycalled a rotating-polarizer or rotating-analyzer ellipsometer, is termed“an incomplete” polarimeter, because it is insensitive to the handednessof the circularly polarized component and exhibits poor performance whenthe light being analyzed is either nearly completely linearly polarizedor possesses a depolarized component. However, using UV light fromsource 90, the substrate of materials such as silicon contribute enoughto the overall phase shift of the light interacting with the sample thataccurate results can be obtained without the use of a compensator. Insuch a case, the same formulas above can be used to derive thickness d,where the phase shift induced by the compensator is set to be zero.

[0058] It is to be understood that the present invention is not limitedto the embodiments described above and illustrated herein, butencompasses any and all variations falling within the scope of theappended claims. For example, beams 24, 26, and/or 106 can betransmitted through the sample, where the beam properties (including thebeam polarization state) of the transmitted beam are measured. Further,a second compensator can be added, where the first compensator islocated between the sample and the analyzer, and the second compensatorlocated between the sample and the light source 90, as illustrated inFIG. 4. These compensators could be static or rotating. In addition, toprovide a static or varying retardation between the polarization states,compensator 98 can be replaced by a non-rotating opto-electronic elementor photo-elastic element, such as a piezo-electric cell retarder whichare commonly used in the art to induce a sinusoidal or static phaseretardation by applying a varying or static voltage to the cell.

What is claimed is:
 1. An optical measurement system for monitoring asample comprising: a first optical path for directing a polychromaticprobe beam substantially normal to the sample surface; a second opticalpath for directing a polychromatic probe beam at an oblique angle to thesample surface; and a common spectrometer for selectively measuringlight reflected from the sample originating from either the firstoptical path or the second optical path and generating output signals asa function of wavelength.
 2. An optical measurement system formonitoring a sample comprising: a first optical path for directing apolychromatic probe beam substantially normal to the sample surface,said first optical path including a focusing element for focusing thenormally directed probe beam onto the sample surface; a second opticalpath for directing a polychromatic probe beam at an oblique angle to thesample surface; and a common spectrometer for selectively measuringlight reflected from the sample originating from either the firstoptical path or the second optical path and generating output signals asa function of wavelength.
 3. An optical measurement system formonitoring a sample comprising: a first optical path for directing apolychromatic probe beam substantially normal to the sample surface,said first optical path including a focusing element for focusing thenormally directed probe beam onto the sample surface; a second opticalpath for directing a polychromatic probe beam at an oblique angle to thesample surface; a common spectrometer for selectively measuring lightreflected from the sample originating from either the first optical pathor the second optical path and generating output signals as a functionof wavelength; and a processor for evaluating the characteristics of thesample based on the output signals.
 4. An optical measurement system formonitoring a sample comprising: a first optical path for directing apolychromatic probe beam substantially normal to the sample surface,said first optical path including a focusing element for focusing thenormally directed probe beam onto the sample surface, with the reflectedlight coming back up through the focusing element; a second optical pathfor directing a polychromatic probe beam at an oblique angle to thesample surface; a third optical path for collecting light reflected fromthe sample originating from the second optical path; a fourth opticalpath arranged to receive reflected light from both the first and thirdoptical paths; and a common spectrometer located in the fourth opticalpath and generating output signals as a function of wavelength
 5. Anoptical measurement system for monitoring a sample comprising: a firstoptical path for directing a polychromatic probe beam substantiallynormal to the sample surface, said first optical path including afocusing element for focusing the normally directed probe beam onto thesample surface, with the reflected light coming back up through thefocusing element; a second optical path for directing a polychromaticprobe beam at an oblique angle to the sample surface; a third opticalpath for collecting light reflected from the sample originating from thesecond optical path; a fourth optical path arranged to receive reflectedlight from both the first and third optical paths; a common spectrometerlocated in the fourth optical path and generating output signals as afunction of wavelength; and a processor for evaluating thecharacteristics of the sample based on the output signals.
 6. An opticalmeasurement system for monitoring a sample comprising: a first opticalpath for directing a polychromeatic probe beam substantially normal tothe sample surface, said first optical path including a focusing elementfor focusing the normally directed probe beam onto the sample surface,with the reflected light coming back up through the focusing element; asecond optical path for directing a polychromatic probe beam at anoblique angle to the sample surface; a third optical path for collectinglight reflected from the sample originating from the second opticalpath; a fourth optical path arranged to receive reflected light fromboth the first and third optical paths; a common spectrometer located inthe fourth optical path and generating output signals as a function ofwavelength, and a processor for evaluating the characteristics of thesample based on a combination of output signals corresponding to lightoriginating from both the first and second optical paths.
 7. An opticalmeasurement system for evaluating the characteristics of a sample, saidsystem comprising: a broadband reflective spectrometer including a firstpolychromatic probe beam focused onto the sample from a normalorientation and a first means for measuring the intensity of thereflected first probe beam as a function of wavelength; and a broadbandspectroscopic ellipsometer including a second polychromatic probe beamfocused onto the sample at an oblique angle of incidence and a secondmeans for measuring the change in polarization state of the reflectedsecond probe beam as a function of wavelength and wherein the first andsecond measuring means includes a single, common spectrometer forgenerating output signals as a function of wavelength.
 8. A measurementsystem for evaluating the characteristics of a sample comprising: awhite light source generating a polychromatic probe beam; a firstoptical element for directing the probe beam along either a first orsecond optical path, wherein the first optical path allows the beam tobe focused generally normal to the sample surface and the second opticalpath allows the beam to focused at an oblique angle to the sample; and asecond optical element for directing light reflected from the sample andoriginating from either the first or second optical path along a commonthird optical path, said common third-optical path including a commonspectrometer for generating output signals as a function of wavelength.9. A measurement system as recited in any of claims 1 to 8, wherein saidspectrometer includes an optical element for angularly dispersing thelight as a function of wavelength and an array of detector elements. 10.A measurement system as recited in any of claims 1 to 6, wherein thepolychromatic probe beam traveling along both the first and secondoptical paths is generated by the same light source.
 11. A measurementsystem as recited in claim 7, wherein the first and second probe beamsare generated by the same source.
 12. A measurement system as recited inany of claims 1, 2 or 8, further including a processor evaluating thecharacteristics of the sample based on the output signals.
 13. Ameasurement system as recited in claim 12, wherein the processorevaluates the characteristics of the sample based on a combination ofoutput signals corresponding to light originating from both the firstand second optical paths.
 14. A measurement system as recited in claim7, further including a processor for evaluating the characteristics ofthe sample based on a combination of output signals corresponding tolight associated with the broadband reflective spectrometer and thebroadband spectroscopic ellipsometer.
 15. A measurement system asrecited in claims 3 or 5, wherein the processor evaluates thecharacteristics of the sample based on a combination of output signalscorresponding to light originating from both the first and secondoptical paths.
 16. A method of monitoring a sample comprising the stepsof: directing a polychromatic probe beam substantially normal to thesample surface so that is reflected therefrom; directing a polychromaticprobe beam at an oblique angle to the sample surface so that it isreflected therefrom; and measuring light reflected from the sampleoriginating from either the first optical path or the second opticalpath using a common spectrometer which generates output signals as afunction of wavelength.
 17. A method of evaluating the characteristicsof a sample comprising the steps of: obtaining broadband reflectivespectrometer measurements of the sample using a polychromatic probe beamfocused onto the sample from a generally normal direction; obtainingspectroscopic ellipsometric measurements of the sample using apolychromatic probe beam focused onto the sample at an oblique angle andwherein both said measurements include using a common spectrometer forgenerating output signals as a function of wavelength.
 18. A system forevaluating the characteristics of a sample by monitoring the changesinduced in a polychromatic probe beam directed to interact with thesample comprising: a first optical element for directing the probe beamalong either a first or second optical path; and a second opticalelement for directing light reflected from the sample and originatingfrom either the first or second optical path along a common thirdoptical path, said common third optical path including a commonspectrometer for generating output signals as a function of wavelength.19. A system as recited in claim 18, wherein said spectrometer includesan optical element for angularly dispersing the light as a function ofwavelength and an array of detector elements.
 20. A system as recited inclaim 18, wherein the polychromatic probe beam traveling along both thefirst and second optical paths is generated by the same light source.21. A system as recited in claim 18 Other including a processorevaluating the characteristics of the sample based on the outputsignals.
 22. A system as recited in claim 21, wherein the processorevaluates the characteristics of the sample based on a combination ofoutput signals corresponding to light originating from both the firstand second optical paths.